Electrostatics are used widely in industry, for instance it is used in solid state electronic devices, crop spraying, spinning of cotton, diagnostic equipment used in medical applications, paint spraying, smoke detectors, laser and inkjet printers, and many more different applications. A more complete overview can be found in Fundamentals of Applied Electrostatics, Joseph Crowley, ISBN 0471803189 (p. 229 to 239). Which reference is incorporated by reference in its entirety. There are well known electrostatic effects in nature such as lightning, and less known effects such as St Elmo's fire, a corona discharge from spars of a ship or from an airplane.
The deposition system in this invention is of the type whereby the atomization of a flowable material is principally obtained by charging it to a high electrostatic charge through direct contact with a conductive strip that is connected to a high voltage power supply.
The electrostatic field exerts a coulombic force on the surface of the flowable material and this is the dominant force for the dispersion process. In “hybrid systems” mechanical forces are used for dispersion whilst an applied electrostatic field, which can be by contact, induction or spraying through ionized air from a corona discharge, ensures that drops are charged.
The fluid dynamic processes are similar regardless of the type of force or forces used for dispersion. See for example Electrostatic Spraying of Liquids by Adrian Bailey, ISBN 0863800750, p. 60, which reference is incorporated by reference in its entirety, therefore the addition of other atomizing means will not be excluded from this invention.
Examples of applications of spraying using electrostatic principles are: coating of surfaces of solids or flexible webs of materials. (U.S. Pat. No. 2,685,536 Starkey et al, U.S. Pat. No. 2,706,964 Ransburg et al, U.S. Pat. No. 3,930,614 Krenkel, U.S. Pat. No. 2,302,289 Bramston-Cook, up to more recent patents such as U.S. Pat. No. 5,980,919 Greenfield et al).
Inkjet printing is another example in which electrostatic spraying is used. (for instance U.S. Pat. No. 4,814,788 Davies and U.S. Pat. No. 3,577,198 Beam). Other examples are in scrubbing (U.S. Pat. No. 4,095,962 Richards), or in chemical and physical processes such as producing powders and other granular materials (U.S. Pat. No. 4,788,016 Colclough et al). The examples are not exhaustive.
Because of the electrostatic principle of operation, no high pressures and atomizing nozzles are in principle necessary. However there are limitations to the materials that can be sprayed this way, as in most systems a fairly low conductivity is required for the flowable material. As an example, a preferable range of 20,000 to 100,000 pico Siemens is mentioned in U.S. Pat. No. 5,980,919 (Greenfield et al.)
By careful electrical insulation of the complete spray system, including the flowable material supply system, it is possible to extend the range of the conductivity limits for electrostatically spraying flowable materials.
Lower conductivity materials can be sprayed because higher voltages can be used than is possible with systems that are not as well insulated. Materials with higher conductivity can still be electrostatically charged when the complete flowable material supply system is electrically insulated. (U.S. Pat. No. 5,628,463 Nakamura)
In the patent literature, many electrostatic deposition systems have been described. In older systems, flowable materials were electrostatically charged in a plurality of points (U.S. Pat. No. 2,685,536 Starkey et all) but in later patents there is a slot, whereby the slot can be fed from a chamber or channel (U.S. Pat. No. 4,749,125, Escallon et al, and U.S. Pat. No. 4,830,872, Grenfell).
The slot may have an insert to ensure proper distribution of flow (U.S. Pat. No. 4,749,125, Escallon et all) and serrations at the slot's exit are mentioned in both the Escallon patent, in U.S. Pat. No. 4,788,016 (Colclough et al) as well as in U.S. Pat. Nos. 5,209,410 and 5,441,204 (Wichmann et al), to achieve a stable flow distribution by providing charge concentrating tips so that liquid is drawn out into ligaments at these tips.
Several patents mention means to achieve equal flow distribution over the length of a nozzle arrangement. In 2,706,964 (Ransberg et al), a combination of a rotating plug, timed pumps and moving flow directing elements is described, to distribute liquid progressively along a discharge member. In an earlier patent (2,695,002, Miller), a helical grooved rotating plug conveys liquid to successive points along a slot. In several patents (3,020,579 O'Connor, 5,209,410 Wichmann, et al, 5,441,204 Tappel et al, and 5,503,336 Wichmann), a hydrodynamic liquid distribution is described that distributes liquid from one inlet point to distinct dispensing points.
In U.S. Pat. Nos. 3,020,579 and 5,503,336, a binary type of distribution is described. In U.S. Pat. No. 5,209,410 triangular shaped chambers are used, while in U.S. Pat. No. 5,441,204 it is a network that systematically branches the fluid flow to a plurality of spaced distribution points.
In several patents, the geometry of the sharp edge where the electrostatic spray originates is described as this is seen as important for obtaining good results for different spray conditions (U.S. Pat. No. 4,814,788 Davies, U.S. Pat. No. 4,830,872 Grenfell, U.S. Pat. No. 5,503,336 Wichmann).
The concentration or increase of the charge in an electrostatic field by positioning an insulating material in front of a conducting electrode, is described in the literature. (Joseph Crowley, p 20.). In U.S. Pat. No. 4,830,872 (Grenfell) this effect is used and specific dimensions are given. (from 0.5 to 4 mm and 1 to 4 mm) for the distance of non-conductive material to the spray tip, in U.S. Pat. No. 4,788,016 (Colclough at all), a similar geometry is shown, but no specific dimensions are given.
In Adrian Bailey at p. 75, and in several patents (U.S. Pat. No. 4,830,872 Grenfell, U.S. Pat. No. 5,503,336 Wichmann), the good dispersion obtained by electrostatic spraying at low volume throughputs are mentioned, the two patents mention respectively 0.5 ml/cm of blade length per minute and 0.006 cc/min per inch of nozzle.
Ligament flow is an important concept and is sometimes mentioned as a factor that allows for even distribution of the flowable material as it atomizes and moves to the target area. (U.S. Pat. No. 4,830,872 Grenfell, U.S. Pat. No. 4,814,788 Davies, U.S. Pat. No. 4,788,016 Colclough at all).
The finest and most mono disperse spray patterns are obtained when ligament flow is obtained. (Adrian Bailey, p. 61, 75, 76, 77).
The distances or wavelength between ligaments is quadratically and inversely related to the electrostatic field applied and is directly related to the surface tension of the flowable material.
The finest and most mono disperse droplet sizes are obtained in ligament flow with low flow rates, spraying flowable materials with a low surface tension, and by using high electrostatic fields. While it is possible to provide for a wide flow range of 0.006 cc/min per inch of nozzle to 30 cc/min per inch of nozzle (U.S. Pat. No. 5,503,336 Wichmann), it is not feasible to maintain the same droplet size through out this range, with only one nozzle arrangement, unless more ligaments are created as the flow is increased. This would imply increasing the electrostatic field with increased flow.
For this reason, depending on the requirements for droplet size and spray quality, parallel nozzle arrangements are needed to satisfy the requirement for a small droplet size and therefore good dispersion and spray quality, at increasing flow rates.
An example of a double spray assembly is shown in FIG. 3 of U.S. Pat. No. 5,209,410 (Wichmann et al).
The current invention provides a number of novel features. It is an object to provide an electrostatically efficient and compact system with multiple spray heads in a relative small space. It is a further object to provide such a system which can be easily adapted for different flow rates and spray configurations, while maintaining good control and giving fine mono disperse sprays. It is a further object to provide ligament flow with very small distances between the ligaments promoted by the geometry and design, and the capability to create high electrostatic fields. The spray system can be used for the deposition of flowable materials on to a substrate or a surface, or in other applications such as mentioned in the literature. Even distribution over the length of a spray, or of several parallel sprays, is enhanced by supplying sections of the spray length with precisely controlled flows.
As a further object, the conductivity range of the flowable materials that can be sprayed is much wider than of any similar equipment as mentioned in the literature because of a design that uses an absolute minimum of conductive parts, in addition to a flowable material supply system that is electrically insulated. With the paths to ground minimized, less electrical power is needed. In practise this means that several parallel sprays can be powered by one high voltage power supply, and lower currents ensure that higher voltages can be maintained, for obtaining finer droplets in the sprays.
The spray system is designed such as to provide substantial dripless start and stop of the spray or sprays. The system can be heated to provide for spraying of higher melting point materials or to lower the viscosity of the flowable material that is sprayed.
The quality of the spray can be monitored by a vision system consisting of one or more cameras connected to a processor that is capable to observe the number of ligaments and their distribution, as the start of the ligaments show up as distinct points under illumination.